Isophthalic acid is produced by a liquid-phase oxidation reaction of m-dialkylbenzenes such as m-xylene. In general, the m-dialkylbenzene is subjected to a liquid-phase oxidation reaction in acetic acid solvent in the presence of a catalyst such as cobalt and manganese or in the co-presence of the catalyst and an accelerator such as a bromine compound and acetaldehyde to obtain a crude isophthalic acid, and then the resultant crude isophthalic acid is purified to obtain the aimed high-purity isophthalic acid.
However, since acetic acid is used as a solvent in the above reaction and the reaction product contains impurities such as 3-carboxybenzaldehyde (3CBA) and m-toluic acid (m-TOL), a high purification technique is required to obtain the high-purity isophthalic acid.
There are known various methods for purifying the crude isophthalic acid obtained by the above reaction, such as a method of dissolving the crude isophthalic acid in acetic acid, water or an acetic acid/water mixed solvent under high-temperature and high-pressure and then subjecting the resultant solution to catalytic hydrogenation, decarbonylation, oxidation or recrystallization, and a method of subjecting a dispersion partially dissolving isophthalic acid crystal to high-temperature immersion treatment.
In both the production of the crude isophthalic acid by the liquid-phase oxidation reaction and the purification thereof, the separation of the isophthalic acid crystal from the resultant slurry is finally needed. When the dispersion medium (first dispersion medium) of the reaction product solution obtained by the liquid-phase oxidation reaction is acetic acid and a different dispersion medium (second dispersion medium) such as water is used in the purification, it is required to first separate the crystals from the reaction product solution and then re-disperse the separated crystals in the second dispersion medium. When the first dispersion medium in the reaction product solution is the same kind as the second dispersion medium for the subsequent purification, a substantial part of the impurities such as oxidation intermediate, for example, 3CBA and m-TOL and coloring substances remain dissolved in the dispersion medium after a high-temperature purifying operation of the reaction product solution of the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and isophthalic acid crystals. If the reaction product solution of the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and isophthalic acid crystals is cooled to about 100° C. while allowing the impurities to be dissolved therein, the impurities are included into the isophthalic acid crystals, thereby failing to obtain the aimed high-purity isophthalic acid. Therefore, it is necessary to conduct the separation at high temperatures under high pressures to separate a high-purity isophthalic acid from the reaction product solution obtained by the liquid-phase oxidation reaction, the starting slurry composed of the first dispersion medium and isophthalic acid crystals or the slurry after the purification treatment.
A centrifugal separation has been most generally used for separating a slurry into crystals and a dispersion medium, which is also extensively used in the separation of the reaction product solution obtained by the liquid-phase oxidation reaction or the starting slurry composed of the first dispersion medium and isophthalic acid crystals.
In the centrifugal separation, the starting slurry composed of the first dispersion medium and isophthalic acid crystals is introduced into a basket which is rotating at a high speed to allow the first dispersion medium to overflow from the upper portion of the basket and allow the crystals to move downwardly. It has been known that this method involves several problems caused by the limitation in the structures and functions due to the operation at high temperatures under high pressures.
Since the rinsing during the centrifugal separation and the rinsing of the separated crystals are difficult in this method, the amount of the first dispersion medium adhering to the crystals increases. Therefore, the centrifugally separated isophthalic acid crystals are made into a slurry by a further addition of a high-temperature fresh solvent, thereby needing an additional separation into the crystals and the dispersion medium. In addition, the high-speed rotation at high temperatures under high pressures necessitates a difficult and complicated maintenance of the centrifugal separator, to increase the production costs.
For example, in the method for producing a high-purity isophthalic acid disclosed in Patent Document 1, a crude isophthalic acid obtained by a liquid-phase oxidation is catalytically hydrogenated and isophthalic acid is allowed to crystallize to obtain a slurry which is then brought into contact with a hot water to exchange the dispersion medium. It is reported that the quality of isophthalic acid which is taken out of the bottom of a tower for replacing dispersion medium is increased by discharging a part of fine isophthalic acid crystals together with the mother liquor of slurry from the top of tower. However, Patent Document 1 is completely silent about the uniform dispersion of the isophthalic acid crystals in the tower for replacing dispersion medium.    [Patent Document 1] Japanese Patent 3269508